This chapter discusses mating biology of basal fungi with emphasis on chytridiomycetes and zygomycetes. The chytrids and zygomycetes are ecologically diverse. Many of them are parasites, on hosts such as plants, algae, invertebrates, or other fungi. These nutritional requirements have precluded their isolation into axenic culture. Both chytrids and zygomycetes are capable of causing diseases in humans and other animals. A number of zygomycete fungi cause human diseases that are particularly difficult to treat with current antifungal agents. Most of the knowledge of sex in basal fungi comes from studies of behaviors of axenic cultures or careful observations made using light microscopy on natural substrates for the fungi. Chytridiomycetes have long been recognized to be divisible into several major groups based on life cycles and sexual mechanisms. Blastocladiales are exceptional among Fungi by having a life cycle with alternation of generations in which a diploid sporophyte can undergo extensive vegetative growth including asexual reproduction. The Zygomycota comprise nine orders, eight of which include species that undergo homothallic or heterothallic sex. Among the Zygomycota, the Mucorales are the best studied: it is generally inferred that similar patterns of mating occur in the other zygomycetes, although this may not be the case. Microsporidia are obligate intracellular parasites of animals (often insects) and humans, characterized by the absence or remnants of mitochondria and the presence of a specialized structure, the polar tube, with which they infect cells.

A phylogenetic tree of the fungal kingdom, illustrating both diversity and ambiguity of relationships, with species discussed in the text listed. While some groups such as the Dikarya are well supported by phylogenetic studies, the relationships at the base of the tree remain ambiguous (represented by the thick line). Previously, taxa in gray have been classified as members of the Zygomycota and taxa in bold have been classified as members of the Chytridiomycota. Tree based on references 31 and 84.

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Figure 24.1

A phylogenetic tree of the fungal kingdom, illustrating both diversity and ambiguity of relationships, with species discussed in the text listed. While some groups such as the Dikarya are well supported by phylogenetic studies, the relationships at the base of the tree remain ambiguous (represented by the thick line). Previously, taxa in gray have been classified as members of the Zygomycota and taxa in bold have been classified as members of the Chytridiomycota. Tree based on references 31 and 84.

Sexual and vegetative structures of zygomycetes and chytridiomycetes. (a through c) Mating and zygospore formation in the homothallic species Radiomyces spectabilis: fusion of undifferentiated gametangia; the arrow indicates septum delimiting one gametangium (a); young zygospore produced between two suspensors, only one of which is appendaged (arrow) (b); mature zygospore (c). (d) Asexual zoosporangium of Chytriomyces hyalinus subtended by filamentous rhizoids. (e) Resting spore of C. hyalinus. Arrows indicate the two empty zoospore cysts which presumably donated nuclei to the resting spore. (f) Thick-walled resting spores of Allomyces arbusculus.

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Figure 24.2

Sexual and vegetative structures of zygomycetes and chytridiomycetes. (a through c) Mating and zygospore formation in the homothallic species Radiomyces spectabilis: fusion of undifferentiated gametangia; the arrow indicates septum delimiting one gametangium (a); young zygospore produced between two suspensors, only one of which is appendaged (arrow) (b); mature zygospore (c). (d) Asexual zoosporangium of Chytriomyces hyalinus subtended by filamentous rhizoids. (e) Resting spore of C. hyalinus. Arrows indicate the two empty zoospore cysts which presumably donated nuclei to the resting spore. (f) Thick-walled resting spores of Allomyces arbusculus.

39. Kuzina,, V., and E.Cerdá-Olmedo.2006.Modification of sexual development and carotene production by acetate and other small carboxylic acids in Blakeslea trispora and Phycomyces blakesleeanus.Appl. Environ. Microbiol. 72:4917– 4922.